1. Field of the Invention
This invention relates to radar, particularly to pulse compression radar.
2. Description of the Prior Art
In the prior art, radar systems are often required to operate at more than one transmit duty cycle in order to provide more nearly optimum performance under variant conditions and missions. At high transmit duty cycles, low peak power is desired from the transmitter. At low transmit duty cycles, high peak power is desired from the transmitter. If comparable range performance is needed from the radar system in both the high and low transmit duty cycle mode using the same transmitter, some form of pulse compression may be used for the low transmit duty cycle mode to lengthen the transmitted signal and thereby raise the average power directed at the target. Unfortunately, conventional pulse compression systems require transmitting for a comparatively long period of time during which the receiver is turned off. Present pulse compression radar systems have two undesirable effects due to the receiver being turned off for a long period of time. The first is the inability of the radar to detect targets at close range and the second is only a portion of the transmitted pulse may be received causing pulse distortion and time sidelobes due to the fact that the receiver is turned off when transmitting. In other words, the target reflected signal is eclipsed or truncated.
In the operation of a conventional pulse compression radar, a linear FM waveform is transmitted for a period of time, T.sub.expanded. During T.sub.expanded, the carrier frequency of the transmitter is varied linearly over a frequency B.sub.t, which defines the basic signal information bandwidth. When a target reflected echo is received, the signal is processed through a matched filter which compresses the target reflected pulse to a width of T.sub.compressed. The target reflected pulse may be compressed up to 1/B.sub.t. To minimize time sidelobes, a frequency weighting function is incorporated as part of the pulse compression circuit or matched filter. The output of the pulse compression circuit is a narrow pulse with a peak amplitude increased over the input pulse amplitude by approximately the compression ratio, T.sub.expanded /T.sub.compressed. The compression ratio may also be expressed as the product of T.sub.expanded and B.sub.t which is also known as the radar system time bandwidth product. For a given noise bandwidth, the signal-to-noise ratio for the radar system is increased by the compression ratio since more power is being directed towards the target. Furthermore, as stated above the basic range resolution of the radar system T.sub.compressed, is determined by the bandwidth of the transmitted signal B.sub.t.
In view of the prior art, it is therefore desirable to have a pulse compression radar which has a very short minimum range and permits the transmitter to operate at up to 50% duty cycle while maintaining excellent time sidelobes of the target reflected echoes despite eclipsing to provide increased radar range performance.